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Growth, Optical, and Electrical Properties of Single-Crystalline Si-CdSe Biaxial p-n Heterostructure Nanowires Y. F. Zhang,† L. P. You,†,‡ X. D. Shan,† X. L. Wei,§ H. B. Huo,† W. J. Xu,† and L. Dai*,† School of Physics and State Key Lab for Mesoscopic Physics, Electron Microscopy Laboratory, and Key Laboratory for the Physics and Chemistry of NanodeVices and Department of Electronics, Peking UniVersity, Beijing 100871, China ReceiVed: June 16, 2007; In Final Form: July 30, 2007
Single-crystalline Si-CdSe biaxial p-n heterostructure nanowires (NWs) have been grown via chemical vapor deposition method and characterized. The Si and CdSe subnanowires have diameters of about 30 and 60 nm, respectively, and grow along the [2h11h] and [0001] directions, respectively. Room-temperature photoluminescence (PL), Raman-scattering, and electrical transport measurements were made on single SiCdSe biaxial heterostructure NWs. Strong CdSe band-edge emission peaked around 710 nm together with a broad emission centered at 600 nm is observed in the PL spectra. Intense sharp longitudinal optical phonon modes from both CdSe and Si are observed in Raman-scattering spectra. The resistivities, carrier concentrations, and carrier mobilities of single CdSe NW and Si subnanowire are estimated. A good rectification characteristic is observed in the I-V curve of Si-CdSe biaxial NW, which confirms that the Si-CdSe biaxial NW is a p-n heterostructure.
Introduction Semiconductor heterostructures, which have a build-in potential at the interfaces, are intimately related to the operating principles of many semiconductor optoelectronic devices, such as light-emitting diodes (LEDs) and laser diodes.1 To date, nanoLEDs2-5 and nano-laser-diodes6 have been successfully fabricated on direct band gap semiconductor nanowires (NWs)/Si heterostructures by taking advantage of both the bottom-up assembly and the top-down planar Si fabrication processes. In order to fabricate high-performance optoelectronic devices, perfect interfaces realized by epitaxial growth are required. To date, various novel one-dimensional (1D) semiconductor heterostructures have been grown.7-10 Furthermore, several novel nano-LEDs11-13 and nano-FETs14 have been made directly on the epitaxially grown semiconductor nanoheterostructures, which provides prospects for uncovering new physics, new devices, and new technology. Silicon is the most important semiconductor in microelectronics. CdSe, with a direct band gap of 1.74 eV at room temperature, is one of the most important materials in making optoelectronic devices.15-17 Indisputably, epitaxial growth of single-crystalline Si-CdSe 1D heterostructures with an ideal heterostructure interface should be of great importance in exploring new ways to realize Si-based optoelectronic integration. Recently, Si-CdSe 1D heterostructures have been synthesized.18,19 However, detailed investigation of the physical qualities of them is still lacking. In this paper, we report the growth, characterization, optical, and electrical properties of single-crystalline Si-CdSe biaxial p-n heterostructure NWs. The results show that these NWs have high crystal, optical, and * Corresponding author: E-mail:
[email protected]. † School of Physics and State Key Lab for Mesoscopic Physics. ‡ Electron Microscopy Laboratory. § Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics.
electrical qualities. The as-synthesized NWs have potential application in making novel nano-optoelectronic devices in the future. Experimental Section The Si-CdSe biaxial heterostructure NWs were grown via chemical vapor deposition (CVD) method. CdSe (99.99%) powders were used as the source, and pieces of p-Si wafer covered with 10 nm thick thermally evaporated Au catalyst were used as the substrates. A quartz boat loaded with CdSe powders and Si substrates, with the CdSe at the upstream of a highly purified Ar carrier gas, was inserted into a quartz tube. The distance between the CdSe powders and Si substrates was 15 to 18 cm. The air inside the quartz tube was driven out by pumping the quartz tube with a rotation pump and backfilling it with the Ar gas for several times. Then with the Ar gas flow rate of about 200 sccm under atmospheric pressure, the vacuumsealed quartz tube was inserted into a tube furnace with an adjustable temperature gradient. The temperatures at the source and the substrates were about 950 and 850 °C, respectively. The growth duration was about 1 h. After the growth, yellowgreen wool-like products could be found on the Si substrates. The as-prepared products were characterized by a field emission environmental scanning electron microscope (ESEM) (Quanta 200 FEG), a high-resolution transmission electron microscope (HRTEM) (Tecnai F30) equipped with an energy-dispersive X-ray spectroscope (EDX), and a high-angle angular dark-field scanning transmission electron microscope (HAADF-STEM). Room-temperature photoluminescence (PL) and Raman-scattering measurements on single biaxial NWs were done with a microzone confocal Raman spectrometer (HORIBA Jobin Yvon, LabRam HR800) equipped with a 40× UV lens and a 100× vis lens. The 325 nm line of a He-Cd laser (Kimmon IK3301RG) and the 633 nm line of a He-Ne laser were used as the excitation sources for PL and Raman-scattering measurements, respectively. Room-temperature electrical transport measure-
10.1021/jp074678d CCC: $37.00 © 2007 American Chemical Society Published on Web 09/12/2007
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Figure 1. (a) ESEM image for as-synthesized Si-CdSe biaxial heterostructure NWs. (b) ESEM image for a single Si-CdSe biaxial NW. (c) SEM image for a single CdSe NW with a Au particle at one end.
Figure 2. (a) HAADF-STEM image of a Si-CdSe biaxial heterostructure NW. (b) The EDX data taken from the biaxial NW depicted in (a). (c and d) The corresponding EDX data taken from the subnanowires with dark and light contrast, respectively. (e) Line-scanning elemental mappings of Cd, Se, Si, and O along the route indicated by the red line in (a).
ments on a Si-CdSe biaxial NW were done by using a nanoprobe system (Kleindiek MM3A) installed in an SEM (FEI XL 30F). The chemically etched tungsten tips were used as the nanoprobes. The nanoprobe system was connected to a semiconductor characterization system (Keithley 4200). Results and Discussions Figure 1a shows an ESEM image for the as-prepared NWs. Figure 1b shows a magnified ESEM image of one such NW. We can see that this NW is formed by two subnanowires (seen more clearly at the end of the NW). Both subnanowires have smooth surfaces and uniform diameters along the axes. It is worth noting that in our experimental condition, besides the SiCdSe NWs, there are also some amounts of CdSe NWs being synthesized. Figure 1c is an SEM image for a CdSe NW. From this figure, we can see clearly a Au particle at one end of the NW. This confirms the growth model for CdSe NWs is based on the well-known vapor-liquid-solid (VLS) mechanism. The samples for TEM observation were prepared by scraping the NWs from the Si substrates, dispersing them in ethanol,
and dropping the suspension solution onto copper grids coated with carbon films. We use the HAADF-STEM technique to characterize the components across the biaxial NW. The HAADF-STEM technique, also called Z-contrast technique (Z is the atomic number), detects the scattered intensity at high angles and forms mostly an incoherent image. The HAADFSTEM image intensity approaches Z2. The HRTEM image, on the other hand, is the bright-field image, which uses the lowangle scattering and forms a phase contrast image. When the observed specimen is thin enough, a dark spot in the HRTEM image should be a light spot in the HAADF-STEM image. Figure 2a shows an HAADF-STEM image of an as-grown biaxial NW. Two different contrasts can be seen clearly through the entire NW. Figure 2b is the EDX data taken from the biaxial NW. The obtained elements include Cd, Se, Si, Cu, and O. The Cu peaks originate from the copper grid. Figure 2, parts c and d, shows the EDX nanoanalysis data recorded from the subnanowires with dark and light contrast, respectively. They reveal that Si element locates at the subnanowire with dark contrast and Cd and Se elements with a Cd/Se atomic ratio close
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Figure 3. (a) HRTEM image around the interface between Si and CdSe subnanowires. (b) A magnified HRTEM image of the Si subnanowire. (c) A magnified HRTEM image of the CdSe subnanowire. (d-f) The SAED patterns corresponding to (a-c).
to the stoichiometry of CdSe locate at the subnanowire with light contrast. This suggests the formation of Si-CdSe biaxial NW. Line-scanning (indicated by the red line in Figure 2a) elemental mappings of Cd, Se, Si, and O were conducted to further investigate the spatial distribution of the atomic contents across the biaxial NW. The results are shown in Figure 2e. The profile of Si is at the left side, while the profiles of Cd and Se are at the right side. Analysis of these data shows that the diameters of Si and CdSe subnanowires are about 30 and 60 nm, respectively, which is consistent with the HAADF-STEM image. A small amount of O distributes almost uniformly along the radial direction. This may result from the unavoidable oxygen adsorption during the TEM sample preparation processing. We think the Si source comes from the Si substrates, since Au and Si can form an alloy at a temperature as low as 363 °C and the atom percentage of Si in the Au-Si alloy at our growth temperature (850 °C) is about 43%. During the growth process, a certain amount of Si atoms may evaporate from the Au-Si alloy and nucleate at CdSe subnanowires, which serve as the preferable absorption sites for the silicon atoms and eventually work as the templates for the growth of Si subnanowires.20 Figure 3a is an HRTEM image around the interface between Si and CdSe subnanowires. We can see clearly the crystal planes of both Si and CdSe subnanowires. The interface of the heterostructure is abrupt with a width of only about 3 nm. For Si subnanowire, the crystal planes with a spacing distance of about 0.31 nm (seen more clearly in the magnified image shown in Figure 3b) are seen parallel to the interface. According to the JCPDS (80-0018) data, these planes can be indexed as Si {111} planes. For the CdSe subnanowire, the crystal planes with spacing distances of about 0.37 and 0.7 nm are seen parallel and perpendicular to the interface, respectively (seen more clearly in the magnified image shown in Figure 3c). According to the JCPDS (77-2307) data, these planes can be indexed as CdSe {011h0} and {0001} planes, respectively. The selected area electron diffraction (SAED) patterns corresponding to Figure 3a-c are shown in Figure 3d-f, respectively. By carefully checking the included angles of the diffraction patterns and their corresponding spacing distances of crystal planes, we can index
the zone axes of Si and CdSe subnanowires to be [011] and [21h1h0], respectively, and the growth directions of them to be [2h11h] and [0001], respectively. Corresponding Miller indices are labeled in the figures. All the above results show that the Si and CdSe subnanowires are single crystalline and they grow heteroepitaxially on each other. This can be easily understood, since there are only about 6% and 12% mismatches between the atoms in the Si (111h) and the CdSe (011h0) planes along the directions parallel and perpendicular to the NW growth direction, respectively. The samples for microzone PL and Raman-scattering measurements were prepared by dropping the NW suspension solution onto the quartz substrates. Figure 4a shows the roomtemperature PL spectrum for a single Si-CdSe biaxial NW. A strong CdSe band-edge emission peaked around 710 nm17 together with another broad emission centered at 600 nm is observed. We think the latter one comes from the luminescence centers located in the native SiOx layer surrounding the Si subnanowire.21 Room-temperature Raman-scattering spectrum for a single Si-CdSe biaxial NW is shown in Figure 4b. Phonon peaks are observed at 206, 412, and 518 cm-1, which can be assigned to the longitudinal optical (LO) phonon mode, the overtone of 2LO phonon mode of CdSe NW,22 and the LO phonon mode of Si NW,23 respectively. In comparison to that for the quartz substrate (red line), the Raman-scattering spectrum for the biaxial NW exhibits an increase of intensity at higher wave number. This phenomenon results from the PL from CdSe. The observed sharp LO phonon modes suggest the as-grown Si-CdSe biaxial NWs have high crystal quality. The sample for electrical measurement was prepared by dropping the NW suspension solution onto a Si substrate covered with a 300 nm SiO2 layer. First, we move the two nanoprobes to contact with a single CdSe NW. The corresponding SEM image and the I-V curve are shown in Figure 5, parts a and b. The I-V curve shows an almost symmetric nonohmic contact behavior, which results from the back-to-back Schottky contacts formed between the tungsten nanoprobes and the CdSe NW. Second, we move the two nanoprobes to contact with a Si subnanowire. This is easy to do, because part of the CdSe
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Figure 4. (a) Room-temperature PL spectrum measured on a single Si-CdSe biaxial NW. (b) Room-temperature Raman-scattering spectrum measured on a single Si-CdSe biaxial NW.
one CdSe subnanowire, and the other one to contact with the Si subnanowire. The nanoprobe contacting with the Si subnanowire was grounded. The SEM image of the biaxial NW contacted by two tungsten probes and the corresponding I-V curve are shown in Figure 5, parts e and f. Unlike the I-V characteristic of single CdSe NW or Si NW, a good rectification characteristic is obtained. This suggests that the two Schottky junctions in the circuit are face-to-back. This only occurs when the conduction types of Si and CdSe are different. In other words, the Si-CdSe NW is a p-n junction. The fact that the forward current is obtained when a positive voltage is applied on the tungsten tip contacting with CdSe means that the CdSe subnanowire is n-type, and hence the Si subnanowire is p-type. This makes sense, since the substrate used in the synthesis is p-type Si. An amount of the carriers injected from the tungsten tips will arrive at the edges of the depletion regions of the reverse-biased p-n junction through diffusion and be accelerated out of the depletion region. This process is similar to the way the carriers from the forward-biased emitter junction inject into the reverse-biased collector junction in an abipolar transistor. Conclusion
Figure 5. (a, c, and e) SEM images of a CdSe NW, a Si subnanowire, and a Si-CdSe p-n junction NW contacted by two tungsten probes, respectively. (b, d, and f) Representative I-V curves corresponding to (a, c, and e) experimental setups.
subnanowire in the biaxial NW is broken off during the sample preparation. The CdSe and Si subnanowires are identified by their contrast and diameters. The corresponding SEM image and the I-V curve are shown in Figure 5, parts c and d. Again, the I-V curve shows an almost symmetric nonohmic contact behavior, which results from the back-to-back Schottky contacts formed between the tungsten nanoprobes and the Si NW. Zhang and co-workers have suggested a metal-semiconductor-metal (M-S-M) model to analyze quantitatively the I-V characteristics of such an M-S-M system.24,25 Using this model, we can reproduce the observed I-V characteristics using a few fitting variables and estimate the intrinsic parameters of the NW. The fitted values of resistivity, electron concentration, and electron mobility of the CdSe NW are (0.042 ( 0.0014) Ω‚ cm, (1.8 ( 0.1) × 1017/cm3, and (825 ( 50) cm2/V‚s, respectively. The fitted values of resistivity, hole concentration, and hole mobility of the Si subnanowire are (0.080 ( 0.015) Ω‚cm, (1.3 ( 0.2) × 1018/cm3, and (60 ( 13) cm2/V‚s, respectively. Finally, we move one nanoprobe to contact with
In conclusion, we have grown Si-CdSe biaxial epitaxial heterostructure NWs with abrupt interfaces via CVD method. ESEM and HAADF-STEM images together with the EDX nanoanalysis indicate that the Si and CdSe subnanowires have diameters of about 30 and 60 nm, respectively. The HRTEM image together with the SAED patterns indicates that the Si and CdSe subnanowires are single crystalline and grow along the [2h11h] and [0001] directions, respectively. Besides, Si (111h) and CdSe (011h0) crystalline planes are parallel to the interface of the heterostructure. Strong band-edge emission from single CdSe subnanowire (∼710 nm) and emission from the luminescence centers located in the native SiOx layer surrounding the single Si subnanowire (∼600 nm) are observed in roomtemperature PL spectra. Sharp and intense LO phonon modes from single CdSe (206 cm-1) and Si subnanowires (518 cm-1) are observed in room-temperature Raman-scattering spectra. The intrinsic parameters of the single CdSe NW and Si subnanowire are estimated by using an M-S-M model. The I-V curve of single Si-CdSe biaxial NW exhibits a good rectification characteristic and confirms that the NW is a p-n heterostructure. All the results show that the as-grown biaxial NWs are with high crystal, optical, and electrical properties, which may benefit the later-on fabrication of novel nano-optoelectronic devices. Acknowledgment. This work was supported by the National Natural Science Foundation of China under Grant Nos. 60576037, 10574008, and 60476023 and the National Basic Research Program of China (Nos. 2006CB921607 and 2007CB613402).
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